Woah, in the video
Hells Malice linked to, it shows so many things:
Abilities (such as
sprint {CoD, anyone?}, that weird shield thing, camo-on-demand)
No more spike and flame grenades
Health bar (which seems to be smaller when you're an Elite)
Jetpacks
Assault rifle with only 22 rounds (f*ck!)
Ye ol' Halo 1 pistol is coming back
New weapons (Needle rifle, the DMR {BR}, some sort of n00bt00b, and that weird plasma rocket thing)
Assasinations (performed by holding the B button, looks like you can use a knife)
New Game modes (Invasion, Headhunter, Spartan vs Elite)
Resizing (blooming) Reticules
Massive amount of people/vehicles on screen
And stuff that wasn't in the trailer:
Full AI
4 times better graphics when compared to Halo 3
Better character animation (mo-cap)
Particle effects (sparks, plasma burning people, dirt flying, better weather, smoke, debris)
Forge/Armor customization returns from Halo 3
Sandbox-style enemy encounter
A roflcopter...erm...helicopter
This game looks amazing.